1. Department of Biomedical Engineering City University of Hong Kong Hong Kong 999077 P. R. China
2. Department of Mechanical Engineering The University of Hong Kong Hong Kong 999077 P. R. China
3. Department of Electrical and Electronic Engineering The University of Hong Kong Hong Kong 999077 P. R. China
4. Center for Advanced Materials (CAM) Heidelberg University 69120 Heidelberg Germany
5. Department of Electrical and Computer Engineering New Jersey Institute of Technology Newark NJ 07102 USA